1. Field of Invention
This invention relates generally to a method and apparatus for detecting the presence or absence of a person on a flexible support, and more particularly, to a mechanism and method to be used with a sling-type seat using an expandable strap.
2. Discussion of Related Art
It is desirable to be able to monitor when a person is either present on or is not present on a flexible support, such as a seat or a bed. A typical example of such a seat is a sling-type of seat which typically includes a fabric seat portion which extends between at least two generally parallel, rigid support members. The fabric is flexible and provides a degree of comfort to the person seated, yet must be sufficiently strong so that it can support the weight of such a person. The seat may be made of any suitable fabric, such as canvass, cotton, nylon, Kevlar or the like. Chairs are typically provided with a sling-type of seat to allow for folding of the chair for storage or transport. One typical example of a chair with a sling-type of seat is a wheelchair.
Because of its non-rigid nature, the seat of a sling-type of seat will sag in the middle. When a person is seated on the seat, the seat, regardless of the material from which it is formed, will stretch slightly, producing a greater sag. The seat will demonstrate some level of sag even when no one is seated therein. With time, the fabric of the sling-type seat will tend to take on a permanent stretch and sag after repeated use. This is commonly termed “sling seat sag”. With respect to a wheelchair, some of this additional sag is due to side supports being pulled toward one another by the weight of the body and part of the sag is due to whatever inherent stretchability is found in the fabric. As this sag increases, the spacing of the bottom of the seat from a horizontal line drawn between the two side supports also increases, and the actual length of the fabric material of the seat is necessarily longer than a straight, generally horizontal line drawn between the two support members.
When a person arises from such a sling-type of seat, the inherent elasticity in the material of the seat will cause the seat to return ever so slightly toward its pre-stretched length. In addition, the side support rails may return to a wider spacing which is maintained by any support structure and braces.
It would be desirable to be able to determine when someone becomes seated in a sling-type seat, particularly a wheelchair. It would also be desirable to determine when someone arises from a sling-type seat, such as a wheelchair. This information could be used to monitor movement of a patient for various purposes.
Currently there exist compression switches that are used in conjunction with wheelchairs and that are activated when a person is seated in a wheelchair. These compression switches, however, are attached to the side rails by an inelastic or rigid member. These types of switches are undesirable because they cause unnecessary pressure from beneath the seat on the person seated therein. This rigid or non-elastic member not only is uncomfortable for a seated person, but causes excessive pressure which could lead to skin breakdown and, after continuous or substantially extended use, a decubitus ulcer.